Chapter 10a Sensory Physiology

General properties of sensory systems Somatic senses About this Chapter General properties of sensory systems Somatic senses Chemoreception: smell and taste The ear: hearing The ear: equilibrium The eye and vision

General Properties: Sensory Division Table 10-1 (1 of 2)

General Properties: Sensory Division Table 10-1 (2 of 2)

Stimulus as physical energy  sensory receptor Sensory Pathways Stimulus as physical energy  sensory receptor Receptor acts as a transducer Intracellular signal  usually change in membrane potential Stimulus  threshold  action potential to CNS Integration in CNS  cerebral cortex or acted on subconsciously

Somatosensory Receptors Stimulus Free nerve endings Unmyelinated axon Cell body (a) Figure 10-1a

Somatosensory Receptors Stimulus Enclosed nerve ending Layers of connective tissue Myelinated axon Cell body (b) Figure 10-1b

Somatosensory Receptors Stimulus Specialized receptor cell (hair cell) Synaptic vesicles Synapse Myelinated axon Cell body of sensory neuron (c) Figure 10-1c

Sensory Receptors Table 10-2

Stimulus energy converted into information processed by CNS Sensory Transduction Stimulus energy converted into information processed by CNS Ion channels or second messengers initiate membrane potential change Adequate stimulus: Preferred form of stimulus Threshold: Minimum stimulus Receptor potential: Change in sensory receptor membrane potential

Receptive Fields of Sensory Neurons Primary sensory neurons The primary sensory neurons converge on one secondary sensory neuron. Information from the secondary receptive field goes to the brain. Secondary sensory neuron The receptive fields of three primary sensory neurons overlap to form one large secondary receptive field. SECTION THROUGH SPINAL CORD Figure 10-2

Sensory Neurons: Two-Point Discrimination Two-point discrimination varies with the size of the secondary receptive field (a) Compass with points separated by 20 mm Skin surface Primary sensory neurons Secondary sensory neurons One signal goes to the brain. Figure 10-3a

Sensory Neurons: Two-Point Discrimination Two-point discrimination varies with the size of the secondary receptive field Two signals go to the brain. Compass with points separated by 20 mm Primary sensory neurons Skin surface Secondary sensory neurons (b) Figure 10-3b

Integration by CNS Sensory information Spinal cord to brain by ascending pathways Directly to brain stem via cranial nerves Visceral reflexes integrated in brain stem or spinal cord usually do not reach conscious perception Perceptual threshold: level of stimulus necessary to be aware of particular sensation

Sensory Pathways Each major division of the brain processes one or more types of sensory information

Sensory Pathways Figure 10-4 Primary somatic sensory cortex Gustatory cortex Olfactory cortex Olfactory bulb Auditory cortex Visual cortex 1 Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex. Eye 2 Cerebellum 2 Most sensory pathways project to the thalamus. The thalamus modifies and relays information to cortical centers. 1 Nose Thalamus Sound Brain stem Equilibrium 3 3 Equilibrium pathways project primarily to the cerebellum. Tongue Somatic senses Figure 10-4

Properties of Stimulus: Modality Indicated by where Sensory neurons are activated Neurons terminate in brain Specific to receptor type Labeled line coding 1:1 association of receptor with sensation

Properties of Stimulus: Location According to which receptive fields are activated Auditory information is an exception Sensitive to different frequencies Lateral inhibition Increases contrast between activated receptive fields and inactive neighbors Population coding Multiple receptors functioning together

Properties of Stimulus: Location The brain uses timing differences rather than neurons to localize sound Figure 10-5

Properties of Stimulus: Location Lateral inhibition enhances contrast and makes a stimulus easier to perceive Stimulus Stimulus Pin Skin A B C Frequency of action potentials Tonic level Primary neuron response is proportional to stimulus strength. Primary sensory neurons Pathway closest to the stimulus inhibits neighbors. Secondary neurons A B C Frequency of action potentials Inhibition of lateral neurons enhances perception of stimulus. Tertiary neurons Tonic level A B C Figure 10-6

Properties of Stimulus Intensity Coded by number of receptors activated and frequency of action potentials Duration Coded by duration of action potentials Some receptors can adapt or cease to respond Tonic receptors versus phasic receptors

Properties of Stimulus Sensory neurons use action potential frequency and duration to code stimulus intensity and duration Transduction site Trigger zone Myelinated axon Cell body Axon terminal Stimulus Amplitude 20 -20 Membrane potential (mV) -40 Threshold Duration -60 -80 (a) Moderate stimulus 5 10 5 10 5 10 Time (sec) 20 -20 Membrane potential (mV) -40 -60 -80 (b) Longer and stronger stimulus 5 10 5 10 5 10 1 Receptor potential strength and duration vary with the stimulus. 2 Receptor potential is integrated at the trigger zone. 3 Frequency of action potentials is proportional to stimulus intensity. Duration of a series of action potentials is proportional to stimulus duration. 4 Neurotransmitter release varies with the pattern of action potentials arriving at the axon terminal. Figure 10-7

Tonic and Phasic Receptors Figure 10-8a

Tonic and Phasic Receptors Figure 10-8b

Somatic Senses: Modalities Touch Proprioception Temperature Nociception Pain Itch

Somatic Senses Pathways 4 4 Sensations are perceived in the primary somatic sensory cortex. 3 3 Sensory pathways synapse in the thalamus. THALAMUS MEDULLA 2 2 Fine touch, vibration, and proprioception pathways cross the midline in the medulla. Fine touch, proprioception, vibration KEY 1 1 Pain, temperature, and coarse touch cross the midline in the spinal cord. Nociception, temperature, coarse touch Primary sensory neuron Secondary sensory neuron Tertiary neuron SPINAL CORD Figure 10-9

The Somatosensory Cortex Figure 10-10

Touch Receptors in the Skin Meissner’s corpuscle responds to flutter and stroking movements. Merkel receptors sense steady pressure and texture. Hair Free nerve ending Free nerve ending of nociceptor responds to noxious stimuli. Free nerve ending of hair root senses hair movement. Hair root Sensory nerves carry signals to spinal cord. Pacinian corpuscle senses vibration. Ruffini corpuscle responds to skin stretch. Figure 10-11

Temperature Receptors Free nerve endings Terminate in subcutaneous layers Cold receptors Lower than body temperature Warm receptors Above body temperature to about 45°C Pain receptors activated above 45°C

Respond to strong noxious stimulus that may damage tissue Nociceptors Free nerve ending Respond to strong noxious stimulus that may damage tissue Modulated by local chemicals Substance P is secreted by primary sensory neurons Mediate inflammatory response Inflammatory pain

Reflexive protective response Nociceptors Pathways Reflexive protective response Integrated in spinal cord Withdrawal reflex Ascending pathway to cerebral cortex Becomes conscious sensation (pain or itch)

Somatosensory Nerve Fibers Table 10-5

Nociceptors: Pain and Itch Histamine activates C fibers causing itch Pain Subjective perception Fast pain Sharp and localized—by A fibers Slow pain More diffuse—by C fibers

The Gate-Control Theory of Pain Figure 10-12a

The Gate Control Theory of Pain Modulation Figure 10-12b

The Gate Control Theory of Pain Modulation Figure 10-12c

Primary sensory neurons Kidney (uncommon stimulus) Referred Pain Skin (usual stimulus) Primary sensory neurons Kidney (uncommon stimulus) Secondary sensory neuron Ascending sensory path to somatosensory cortex of brain (b) Figure 10-13b

Chronic pain is a pathological pain Analgesic drugs Ischemia Lack of adequate blood flow Chronic pain is a pathological pain Analgesic drugs Aspirin Inhibits prostaglandins and slows transmission of pain to site of injury